In recent years, an increase in the modulation speed and an increase in the configuration scale of an optical module such as an optical modulator have been in progress along with an increase in the capacity of an optical transmission system. Therefore, in an optical transmitter with an optical module mounted therein, it is desirable that a plurality of Mach-Zehnders forming optical waveguides be integrated in a single chip in order to achieve a size reduction. In an optical module, optical waveguides are formed in parallel with one another by four Mach-Zehnders, for example. Two signal electrodes and two ground electrodes are patterned on each of the optical waveguides. The optical module generates a multilevel-modulated signal by inputting electric signals different from each other to the two signal electrodes. In such an optical module, all the electric signal input units are disposed on one side of a package in order to facilitate the mounting of the input unit (such as a coaxial connector) and to reduce the mounting area thereof.
In the optical module with the input units disposed on one side thereof, an electric signal such as an RF (Radio Frequency) signal is inputted thereto via a coaxial connector provided on the side surface of the package. Moreover, a coaxial adaptor for inputting an external electric signal is connected to the coaxial connector. The optical module, however, needs to increase a pitch between the signal electrodes to which electric signals are inputted according to the width of the coaxial adaptor. Thus, when the number of channels is increased, the mounting area is correspondingly increased.
In order to suppress the aforementioned increase in mounting area, a surface-mount optical module in which an electric signal is inputted from a PCB (Printed Circuit Board) side via an FPC (Flexible Printed Circuit) provided in a package has been developed. In such an optical module, an electrode pattern on the PCB and an electrode pad on the FPC are connected to each other with a solder in order to input an electric signal thereto. This eliminates a need for the coaxial adaptor. Thus, a pitch between the signal electrodes to which electric signals are inputted can be reduced, thereby reducing the mounting area thereof. As a result, a reduction in the size of the optical transmitter can be achieved.
In an optical module handling high-frequency signals such as an optical modulator, however, if an impedance mismatch is generated at a connection between an electrode pattern on a PCB and an electrode pad on an FPC, the reflection of a high-frequency signal is increased, thereby deteriorating the high-frequency characteristics thereof. In order to prevent this, it is desirable that the optical module be designed so that the characteristic impedance of each signal electrode at the connection approximately equals an ideal value of 50Ω.
Patent Document 1: Japanese Laid-open Patent Publication No. 2007-123744 and Patent Document 2: Japanese Laid-open Patent Publication No. 2012-182173 are introduced as the Rerated Art Document.
Two ground patterns are formed in parallel to each other on a rear surface of an FPC (a side adjacent to a PCB) so as to interpose one signal pattern therebetween. While a width of the signal pattern is set so that the characteristic impedance at the aforementioned connection equals 50Ω, the width varies depending on a component part thereof. More specifically, an electrode pad portion of the signal pattern has a width of about several hundred μm since the electrode pad portion is soldered to the electrode pattern on the PCB. A microstripline (MSL) portion extending toward a package side, on the other hand, has a width of only about several ten to 100 μm since no soldering is made.
In view of this, the microstripline portion of the aforementioned signal pattern is covered by a cover material (for example, a coverlay) in order to prevent the separation thereof or the like. In order to protect the microstripline and obtain the characteristic impedance of 50Ω, it is desirable that an end of the cover material on the electrode pad side coincide with a boundary between the electrode pad and the microstripline. However, the cover material tends to vary from product to product since the positional accuracy of outer shape machining or bonding in a manufacturing process of the cover material is generally as low as about several hundred μm. Therefore, there is a risk that part of the microstripline is not covered by the cover material due to a manufacturing error thereof such as tolerance and therefore is exposed. Such an exposure of the microstripline becomes a factor of disconnection due to the separation thereof.